The invention relates generally to the field of metal matrix composites. More specifically, the invention relates to methods for providing homogeneous metal matrix composite powders and using the powders as the feedstock for cold spray deposition to produce high quality composite coatings and freestanding bulk forms.
A metal matrix composite (MMC) is a type of composite material having at least two constituent parts, the matrix and a reinforcement. The matrix is typically a metal. The reinforcement may be a different metal or another material such as a ceramic or organic compound.
The matrix is a continuous frame into which the reinforcement is embedded. A path exists through the matrix to any point in the material. The matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement. The materials can remain in an elemental form, in different alloy forms, and in mixtures of two or three different elements, their alloys and compounds. The matrix typically has a higher volume percentage in the composite.
The reinforcement is usually a ceramic. The ceramic may be carbides, nitrites, borides, oxides, or mixtures thereof, and be in either continuous form or discontinuous particulates. The ceramics are added to the matrix to enhance the strength, wear resistance, friction coefficient, thermal conductivity, and other factors.
Continuous reinforcement uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are directionally embedded into the matrix, the result is an anisotropic structure in which the alignment of the material affects its strength.
Discontinuous reinforcement is isotropic and can be worked using standard metalworking techniques. Discontinuous reinforcement uses short fibers or particles. The most common reinforcing materials in this category are alumina and silicon carbide.
MMC manufacturing can be broken into three types: solid, liquid, and vapor. Solid-state methods include powder blending and consolidation, and foil diffusion bonding. Powder blending and consolidation (powder metallurgy) is where powdered metal and discontinuous reinforcement are mixed and then bonded through a process of compaction, degassing, and thermo-mechanical treatment using, as one example, hot isostatic pressing (HIP) or extrusion. Foil diffusion bonding is where layers of metal foil are sandwiched with long fibers, and then pressed through to form a matrix.
Liquid-state methods include stir casting, squeeze casting, spray deposition and reactive processing. Stir casting is where discontinuous reinforcement is stirred into molten metal, which is allowed to solidify. Squeeze casting is where molten metal is injected into a form with fibers preplaced inside of it. Reactive processing involves a chemical reaction with one of the reactants forming the matrix and the other the reinforcement. Spray deposition is where molten metal is sprayed onto a continuous fiber substrate.
Vapor-methods use physical vapor deposition to coat fibers passed through a thick cloud of vaporized metal where the fibers are coated.
Metal matrix composites have found many applications in various industries. Aluminum and titanium alloys reinforced with ceramic fibers and particles have high specific strength values (high strength to weight ratios) and have found acceptance in many aerospace applications. Carbide reinforced metal coatings and bulk forms have exceptional wear resistance and are used in many tribological applications. However, many problems still exist in the preparation of industrial components using these MMC materials.
Thermal spray techniques such as flame, high velocity and plasma, that are performed in an open air environment lead to oxidation of the metal powders. Oxidation results in an unacceptable composite bulk form.
Delamination (bond failure) at the matrix-particulate interface may arise due to improper selection of particle sizes, blending techniques or consolidation procedure. Catastrophic failure of the MMC may occur in the operational environment.
During consolidation processing, decomposition, preferential evaporation and degradation of one or the other component may occur. These reactions result in poor quality MMCs with inferior performance characteristics.
Spray coating a tungsten carbide-cobalt powder leads to decarborization and matrix dissolution. This results in a less hard coating. Consolidation techniques can also cause incorporation of inclusions such as oxides during the consolidation process. Improperly prepared composite powder, coating or bulk form not only leads to loss of strength, wear resistance, and other attributes, but adversely affect the thermal and mechanical properties of the material.
It is a challenge to form high performance MMCs as freestanding bulk forms.